Method For Doping A Polymer

ABSTRACT

A method for forming a conjugated polymer which is doped by a dopant includes the steps of (a) adding a doping agent comprising a dopant moiety to a solution containing the conjugated polymer or a precursor thereof and, optionally, a second polymer, the dopant moiety being capable of bonding to the conjugated polymer, precursor thereof or the second polymer; (b) allowing the dopant moiety to bond to the conjugated polymer, precursor thereof or the second polymer to perform doping of the conjugated polymer, wherein the amount of doping agent added in step (a) is less than the amount required to form a fully doped conjugated polymer.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 09/958,257, filedFeb. 19, 2002, which is the U.S. national phase of InternationalApplication No. PCTIGB00/01288, filed Apr. 5, 2000, the entirerespective disclosures of which are hereby incorporated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for doping a conjugated polymer.Polymers preparable according to the method of the invention areprovided.

2. Description of Related Technology

Doping of conjugated polymers (polymers with pi-conjugated backbonestructures and/or pi-conjugated pendant groups) with strong protonicacid (p-doping) or strong oxidizing (p-doping) or reducing agents(n-doping) is well established in the literature. However, the dopingproceeds readily to completion in the presence of a stoichiometric orexcess amount of dopants. The chemical driving force for maximum dopingis very high, so that it is difficult to arrest the doping level at anintermediate value. The system achieves the maximum doping with about10-50% of the conjugated repeat units doped depending on the polymersystem. For poly(p-phenylenevinylenes) and polyacetylenes, this istypically 10-20%; for polythiophenes, 20-30%; for polyanilines, 40-50%.This maximum level of doping imparts a high level of electricalconductivity of the order of 1-1000 S/cm to the polymers, depending onthe nature and type of the polymers and dopants used, so that theybecome conducting polymers in the process. The bulk carrierconcentration is then roughly of the order of 10²⁰/cm³ to 10²¹/cm³.

However, this high level of doping is unnecessary or even undesirablefor some applications. For example, for a 1-μm thick film (which istypical of the vertical thickness of photonic structures) having aconductivity of 10⁻⁶ S/cm, only a modest 1-V potential difference isrequired to drive a practical device current density of 10 mA/cm²through the film thickness direction. Therefore, film conductivities ofthe order of 10⁻⁶-10⁻² S/cm (typical of the semiconducting range) arealready sufficient for these films to be employed in semiconductingphotonic structures such as distributed Bragg reflectors and waveguides.

Furthermore, when the films are doped to the maximum, such as achievedby straightforward exposure to strong acids or oxidants, their opticalproperties change in drastic ways owing to the formation of new sub-gaptransitions that change the refractive indices of the films and causeparasitic absorption of any emitted light. Both these factors are notdesirable or acceptable for photonic applications. Therefore control ofthe bulk carrier concentration between 10¹⁷/cm³ to 10²⁰/cm³, at anintermediate doping-level at least about one order of magnitude lessthan the maximally-doped case, is crucial.

Applied Physics Letters, volume 73, Number 2, pages 253-255 (1998)reports a study of the Hall mobility and the carrier concentration of aconjugated polymer, namely polythiophene, as a function of theelectrochemical doping level. The doping level of the polymer is changedby varying the oxidation potential i.e. by potentiometric control.

Synthetic Metals, 68, pages 65-70 (1994) is concerned with field-effectmobility and conductivity data obtained from two different amorphousorganic semiconductors which can be doped to a range of differentconductivities.

Synthetic Metals, 89, pages 11-15 (1997) investigates the doping andtemperature dependence of the conductivity of poly(p)-phenylene vinylene(PPV).

Synthetic Metals, 55-57, pages 3597-3602 (1993) investigates electricalconductivity of αα′-coupled dodecathiophene as a function of both dopantlevel and time.

Synthetic Metals, 30, pages 123-131 (1989) discloses a relationshipbetween acid strength and ionization potential of a conjugated polymerthat will give a highly conductive doped complex.

Applied Physics Letters, volume 72, pages 2147-2149 (1998) describes adoped hole transporting polymer. Differing levels of doping are realizedby adjusting the co-evaporation rates of polymer and dopant material.

The methods used to achieve different levels of doping in the abovesystems are not satisfactory for controlling the doping level to such adegree so that a balance between optical and electrical property of thedoped polymer can be struck.

SUMMARY OF THE INVENTION

In view of the above, there remains a need to develop a method forpreparing polymers which are doped to a controlled, low or intermediatelevel which is both simple and cost effective. It is envisaged thatpolymers doped to such a level will be particularly useful in devicessuch as those referred to below in order to avoid the disadvantagesassociated with polymers that are doped to a high level. Thesedisadvantages include intense sub-group absorptions, changes in theoptical properties of the polymer and degradation of the photonicstructure of the polymer. Using polymers that are doped to a controlled,low level or intermediate it will be possible to strike a balancebetween optical and electrical properties of an organic semiconductorwhen used in an optoelectronic device.

The invention aims to provide a method for forming a conjugated polymerthat is partially doped. The invention further aims to provide a polymerpreparable according to the method of the invention and uses of suchpolymers.

Accordingly, the invention provides a method for forming a conjugatedpolymer which is doped by a dopant comprising the steps of:

(a) adding a doping agent comprising a dopant moiety to a solutioncomprising the conjugated polymer or a precursor thereof and,optionally, a second polymer, the dopant moiety being capable of bondingto the conjugated polymer, precursor thereof or the second polymer;

(b) allowing the dopant moiety to bond to the conjugated polymer,precursor thereof or the second polymer to perform doping of theconjugated polymer, characterised in that the amount of doping agentadded in step (a) is less than the amount required to form a fully dopedconjugated polymer.

The invention further provides a conjugated polymer that is doped to acontrolled, low or intermediate level which is preparable according tothe method of the invention.

The invention still further provides a photonic device including apolymer according to the invention.

One embodiment of the invention provides a method for forming apartially doped polymer material, comprising: adding a doping agent tothe polymer or a precursor thereof, the doping agent being capable ofbonding to the precursor or the polymer chain; and causing the dopingagent to leave the precursor or the polymer chain to form a dopantcapable of doping the polymer chain; wherein fewer moles of the dopingagent are added than would be numerically sufficient to fully dope thepolymer chain. Also, the invention provides a partially doped polymermaterial formed by that method. Further, the invention provides adevice/structure (such as a photonic device) that includes such amaterial.

The conjugated polymer or its precursor is:

(i) derivatized with a controlled concentration (typically at the levelless than 10-20% of the amount required for full doping) of a dopantmoiety(ies) or its(their) precursor form(s); or

(ii) blended together with a polymer partner (the second polymer), whichmay or may not be a conjugated polymer itself, which is derivatized withsuch moieties to give the equivalent dopant concentration.

Photonic structures are then fabricated from the partially doped polymermaterials, including higher-order blends and composites, containingthese modified conjugated polymers by film-forming techniques. Asubsequent thermal, irradiation or chemical activation step may berequired to generate the active dopant to dope the conjugated polymer.

In a first aspect of the invention, a method is provided to manipulate aprecursor polyelectrolyte to give a controllable partially-dopedconjugated polymer after elimination. The method involves replacement ofa fraction of the counter-anions of the precursor polyelectrolyte byacid anions, such as sulfonates, phosphonates, phosphates, etc., ofbenzene, naphthalene and other organic derivatives while the precursorpolyelectrolyte is in solution. These anions are converted duringthermal elimination to the corresponding strong organic acids which areless volatile and more compatible with the conjugated polymer than theconventional anions, such as chloride, bromide and acetate. This leadsto a higher retention of a strong acid that could favorably dope thepolymer.

In a second aspect of the invention, a method is provided for controlover partial doping of a host conjugated polymer by blending withmeasured amounts of another substantially-miscible polymer (the “secondpolymer”) which is derivatized with a small fraction of dopant groupssuch as sulfonic acid, phosphonic acid or their precursors. The secondpolymer provides a means to distribute substantially homogeneously acontrolled amount of dopant groups into the host conjugated polymermatrix. For this to occur, the second polymer must be co-soluble in thesame solvent used to deposit the desired conjugated polymer film, andpreferably not undergo phase segregation in the matrix. This can beachieved by derivatizing to form a doped second polymer with a smallfraction (usually less than 50 mol %) of the dopant groups. If thederivatization reaction is carried too far, the material produced tendsto be no longer soluble in the common hydrocarbon solvents used tosolubilize the conjugated polymers because of strong interaction of thepolar dopant groups.

In a third aspect of the invention, a method is provided for controlover partial doping of a host conjugated polymer in solution byderivatization with measured amounts of the dopant moieties, such assulfonic acid, phosphonic acid or their precursors thereby, in effect,creating a copolymer. The order of the reaction may be inverted. Eitherthe polymer can be formed first and then derivatized with a small molefraction of the dopant; or the monomer could be derivatized first withthe dopant group or its precursor and then incorporated at a small molefraction into the primary conjugated polymer. The aim is to distributesubstantially homogenously a controlled amount of dopant groups into theconjugated polymer matrix.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to theattached drawings in which:

FIG. 1 shows the reaction scheme according to Example 1. The scheme isapplicable to conjugated polymers that are fabricated by thermalelimination of a cationic precursor polyelectrolyte. An example of sucha polymer is PPV. In FIG. 1, X is an optional alkyl or aryl spacergroup, OR′ and OR″ are optional alkoxy groups, D is a precursor dopantmoiety, for example, PO₃H⁻, SO₃ ⁻ or OPO₃H⁻, y≦0.05 and n≧10.

FIG. 2 a shows a polymer blend preparable according to the reactionscheme of Example 2. Example 2 is generally applicable to solubleconjugated polymers. Examples of such polymers include alkyl- andalkoxy-derivatives of PPV, poly(fluorenes) and their copolymers. “A” isa conjugated polymer host that is blended with “B”, the second polymer,that is conjugated or non-conjugated. D is a non-precursor dopant moietysuch as PO₃H₂, SO₃H or CF₂COOH and n is as defined for FIG. 1.

FIG. 2 b shows two copolymers preparable according to the reactionscheme in Example 3. Example 3 is generally applicable to solubleconjugated polymers. Examples of such polymers include alkyl- andalkoxy-derivatives of PPV, poly(fluorenes) and their copolymers. OR′,OR″, X, D, n and y are as defined for FIGS. 1 and 2 a.

FIG. 3 shows two reaction schemes for activating a precursor dopantmoiety, D, that is (a) bonded to a second polymer in a blend or (b) partof a co-polymer system. D suitably may be SO₃H and D′ suitably may be

OR′, OR″, y, n and X are as defined for FIG. 1.

FIG. 4 shows a reaction scheme for activating a precursor dopant moietyD′ that is part of a co-polymer system. Suitably D′ may be

and D may be

OR′, OR″, y, n and X are as defined for FIG. 1.

FIG. 5 shows a reaction scheme for activating a precursor dopant moietyD′ that is part of a co-polymer system. Suitably D′ may be SO₃R or PO₃R₂and D may be SO₃H or PO₃H₂. R is a leaving group. OR′, OR″, y, n and Xare as defined for FIG. 1. PAG, a photoacid generator, suitably is adiaryliodonium salt, triarylsulfonium salt or other onium salt.

FIG. 6 shows an example of a polymer according to the inventionincluding a dopant moiety that is a redox group. Suitably D′ may beferrocenium or viologen. OR′, OR″, X, n and y are as defined in FIG. 1.

FIG. 7 shows schematically the energy level structure of a DBR formed bymeans of partially doped conjugated polymer materials. 1 a and 1 b areelectrodes. 2 is an emissive layer. 3 a-3 f are polymer layers whichform a “mirror structure”. The HOMO and LUMO levels of the polymerlayers 3 a-3 f are shown.

DETAILED DESCRIPTION

The conjugated polymer according to the invention may be a homopolymer,copolymer, blend or composite material of conjugated polymers. For thepurposes of the invention, a conjugated polymer is defined as a polymerwith a partially or fully π-conjugated backbone and/or π-conjugatedpendent groups. The conjugated polymer after doping is preferablypartially conducting. The conjugated polymer after doping is preferablypartially semi-conducting.

The invention realizes the incorporation of an amount of dopant, or itsprecursor, into a polymer film, via chemical derivatization on theconjugated polymer chain itself or chemical derivatization on anotherpolymer partner (the second polymer) with which the first polymer is toblended or copolymerized. The second polymer may or may not beconjugated.

The dopant level of a polymer produced according to the method is stableand readily controllable.

Preferably, the amount of doping agent added in step (a) is in the rangeof less than 10-20%, preferably 1 to 10%, of the amount required to forma fully doped conjugated polymer. Accordingly, the polymer produced inaccordance with the method, preferably, is doped to a level of less than10-20% compared to full or maximum doping. More preferably, the amountof doping agent added in step (a) is sufficient to form a 0.001% to 5%doped conjugated polymer. Even more preferably, the amount of dopingagent added in step (a) is sufficient to form a 0.1 to 1% dopedconjugated polymer. The polymer produced in accordance with the method,preferably, will have one doped site per 10²-10⁴ repeat units.

The conductivity of the polymer after doping is suitably more than 10⁻⁹S/cm and less than 1 S/cm, preferably 10⁻⁸ S/cm to 10⁻³ S/cm, orpreferably 10⁻⁶ to 10⁻² S/cm. The conductivity of the polymer afterdoping is suitably less than 1 S/cm. Preferably, the conductivity of thepolymer after doping is less than 10⁻² S/cm or 10⁻³ S/cm. It isenvisaged that the conductivity of the polymer after doping also may beless than 10⁻⁴ S/cm or 10⁻⁵ S/cm. It is envisaged that the conductivitymay be in the range from 10⁻⁹ S/cm to 10⁻¹³ S/cm, and preferably, in therange from 10⁻⁹ S/cm to 10⁻³ S/cm or 10⁻⁶ S/cm to 10⁻² S/cm. The amountof dopant that is added is preferably an effective amount to achieve aconductivity in such a range.

In an alternative definition of a polymer preparable in accordance withthe method, the polymer is doped to a level of 10¹⁷-10¹⁹ cm⁻³.

The doping agent may bond to the precursor or polymer chain by replacinga leaving group on the chain.

In one embodiment of the invention, the doping agent is a protonic aciddoping agent. Suitable dopant moieties include a phosphonic acid group,a sulphonic acid group, a fluoroalkyl carboxylic acid group or an indenecarboxylic acid group.

Where the dopant moiety comprises a precursor to the dopant, theprecursor preferably comprises phosphonate or sulfonate.

If the dopant is incorporated in the precursor form, subsequentactivation to generate the active dopant to dope the polymer may benecessary. This activation could be by thermal, irradiation, chemical orother means. Obviously, this would be unnecessary if the dopant isalready incorporated in the active form.

In a further aspect of the invention, the method also includes a step ofcausing the dopant to dissociate, for example by the application oflight and/or heat. This involves cleaving the dopant moiety from theconjugated polymer, precursor thereof or the second polymer after thedopant moiety has been allowed to bond to it in step (b). Thisadditional step enables the dopant to be dispersed uniformly orsubstantially uniformly into the polymer matrix comprising theconjugated polymer or precursor thereof and, optionally, the secondpolymer. This renders the dopant substantially non-diffusing andnon-volatile.

Where the dopant moiety bonds to a precursor of the conjugated polymer,the method further comprises a step of forming a conjugated polymer fromthe precursor thereof.

The step of causing the doping agent to leave the polymer chain may beeffected by heating. The step of causing the doping agent to leave thepolymer chain may result also in conjugation of the polymer and/orformation of the polymer from the precursor.

Optionally, the dopant moiety may bond to the conjugated polymer,precursor thereof or the second polymer in step (b) via spacer group X.Suitable spacer groups include, but are not limited to, alkyl and arylgroups.

The conjugated polymer according to the invention may be any conjugatedpolymer and, suitably, may comprise precursor polyelectrolyte PPV orsubstituted PPVs. The dopant moiety counterion Y may comprise aphosphonate, sulfonate, phosphate, antimonate, borate, molybdate. Thesecounterions may form a side-chain attached to the polymer backbone whenthey bond to the precursor conjugated polymer.

Non-precursor acid dopants include (a) phosphonic acid, (b) sulfonicacid, (c) fluorocarboxyl acid groups.

Precursor acid dopants include (a) o-nitrobenzyl sulfonate side-chains(converting to sulfonic acid group upon irradiation), see FIG. 3; (b)diazonapthaquinone sulfonate side-chains (converting to indenecarboxylic acid upon irradiation), see FIG. 4; (c) phosphonate orsulfonate ester side-chains, together with an incorporated photoacidgenerator (PAG) such as one of the onium salts (the onium saltsgenerating a strong protonic acid upon irradiation which cleaves thesulfonate or phosphonate ester leaving group to the corresponding acid),see FIG. 5. See for example, A. Reiser “Photoreactive polymers: thescience and technology of resists,” John Wiley & Sons, New York, 1989.

The conjugated polymer according to the invention may comprisepoly(alkylthiophenes) and poly(alkylfluorenes) or their partners. Theyshould be derivatized with a dopant moiety Y comprising a protonic acidgroup. Suitable photonic acid groups include phosphonic acid, sulfonicacid, carboxylic acid or their precursor in the form of esters,anhydrides, azides, hydrazides, amides, acid chlorides. The precursorform is capable of converting to the active protonic acid form underirradiation, thermal exposure, or by reaction with another chemicalagent that may be originally deposited with the film or subsequentlyintroduced to the film. In addition, the acid group or its precursorform could be spaced from the polymer main-chain by an alkyl or arylspacer and could also be attached as a separate functional unit on thepolymer chain.

A further class of suitable dopant moieties for use in the methodinclude redox groups based on TCNQ, DDQ, TTF, ferrocene, viologen,iron(m) chelates, or their precursors. The precursor form is capable ofconverting to the active form under irradiation, thermal exposure, or byreaction with another chemical agent. These groups could also be spacedfrom the polymer main chain by an alkyl or aryl spacer or attached toseparate functional units on the chain. The redox group could acceptelectrons or donate electrons to the conjugated units, thereby p-dopingand n-doping the conjugated units, respectively.

Partially doped materials formed, for example, as described above couldbe used to form photonic structures such as distributed Bragg reflectors(potentially pumped reflectors), confinement heterostructures etc. Someexamples of device structures which particularly advantageously mayinclude a partially doped conjugated polymer preparable according to themethod are described in our co-pending UK patent application number9815271.3, the contents of which are incorporated herein by reference.

The photonic device may include a plurality of layers of such dopedmaterials, the layers alternative in their levels of doping. The devicecould be a mirror, for instance a distributed Bragg reflector.

A distributed Bragg reflector (DBR) consists of a stack of regularlyalternating higher- and lower-refractive index dielectrics (lighttransmissive materials) fabricated to fulfill the Bragg condition forreflection at particular wavelengths. This occurs when the optical pathof the periodicity in the dielectric stack corresponds to half awavelength, and the reflectivity is further optimized when the DBR stackobeys the equation: ½λ=n₁d₁+n₂d₂ and for the best performance, the DBRstack obeys the equation: ¼λ=n₁d₁+n₂d₂,

where n₁, n₂ are the respective refractive indices; d₁, d₂ are thecorresponding component film thicknesses in the DBR; and λ is thedesired reflection wavelength.

FIG. 7 shows schematically the energy level structure of a DBR formed bymeans of partially doped conjugated polymer materials. The doping of thematerials is controlled so that the HOMO or LUMO levels (depending onwhether the mirror is between the anode or the cathode and the emissivelayer) of alternate layers of the mirror are at least approximatelyaligned so that the passage of holes/electrons through the mirror is notsignificantly impeded at the boundaries between layers of the mirror.The thicknesses of the layers of the mirror are chosen to satisfy theconditions for reflection. The refractive index of the layers is relatedto their band gaps, but by means of partial doping the HOMO/LUMO levelscan be aligned independently of the band gaps.

Since the DBR is formed of conjugated material it could beelectrically-pumped to generate photons in addition to reflecting.

The invention will now be described by way of non-limiting illustrativeexamples.

EXAMPLES Example 1 Partial Doping of poly(p-phenylenevinylene) by AnionExchange of the Precursor poly(p-xylylene-alpha-tetrahydrothiophene)Route

This is an exemplification of the scheme outlined in FIG. 1.

This Example illustrates the first aspect of the invention.

Example 1A Preparation of Partially Doped PPV

To effect replacement of chloride by phenylphosphonate: 10 mL of 3 w/v %poly(p-xylylene-alpha-tetrahydrothiophenium chloride) (pre-PPV-Cl) (1.3mmol repeat unit) dissolved in methanol is mixed with 10 mL of 20 w/v %phenylphosphonic acid (13 mmol) dissolved also in methanol. The mixtureis then dialyzed against pure methanol through a dialysis membranehaving molecular weight cut-off of 12,000. This gives apoly(p-xylylene-alpha-tetrahydrothiophenium phenylphosphonate) precursor(pre-PPV-PA) polymer which is retained by the dialysis membrane. Theretentate can then be concentrated to the desired concentration andblended with the parent poly(p-xylylene-alpha-tetrahydrothiopheniumchloride) in the desired ratio for solution casting. This allows controlover the level of doping in the final product.

To analyze the material:

(1) A small volume of the methanol solution pre-PPV-PA was evaporated togive a white solid. Thermogravimetry experiments under nitrogen showthis material exhibits a weight loss step extending from 150° C. toabout 300° C. This occurs over a wider range than for the parentpoly(p-xylylene-alpha-tetrahydrothiophenium chloride) material whichexhibits a weight loss step ending at about 200° C. This is despite thefact that phenylphosphonate, being a better leaving group than chloride,undergoes slow elimination from the PPV backbone event at roomtemperature. The greater thermal stability in thermogravimetryexperiments is therefore due to the considerably lower vapor pressure(and hence smaller evaporation loss) of phenylphosphonic acid. Thisconfirms that greater retention of the phenylphosphonic acid occurs forthe same temperature.

2) The pre-PPV-PA material in methanol is spin-cast onto glasssubstrates and then baked at 18020 C. under vacuum for 2 hours to effectconversion to the conjugated PPV. X-ray photoelectron spectroscopyconfirms a significant retention of the PA: 7 mol % (relative to PPVrepeat unit) retention of the PA compared to less than 0.5 mol %retention of Cl in the parent material.

(3) Photothermal deflection spectroscopy of a film of a precursor PPVcounterbalanced by 10 mol % PA+90 mol % Cl (made by blending) showssub-gap absorption strength of 60 cm⁻¹ at 750-nm wavelength. Thiscorresponds to a doping level (that is, the ratio of ionizeddopant-to-PPV repeat unit) of 0.1 mol %−0.01 mol %.

Example 1B Diode Structure Including Partially Doped PPV

To demonstrate the improvement in electrical conductivity associatedwith partial doping according to the invention diode structures withindium-tin oxide/poly(3,4-dioxy-thiophene):poly(4-styrenesulfonate)composite anodes and calcium cathodes were fabricated for active layers(i) and (ii) defined in Table 1.

TABLE 1 Drive voltage Drive voltage required to required to achievecurrent achieve current density of density of Device structure 1 μA/cm²10 μA/cm² (i) Undoped 69-nm-thick PPV film 5.0 V  7 V with 50 vol %silica (ii) Partially-doped PPV stack having 5.0 V 10 V a combinedthickness of 207 nm for the films with 50 vol % silica and 171 nm forthe neat films, and with an addition 105-nm-thick emitter polymer film

For the structure (i) with the undoped PPV polymer (dispersed withsilica particles to change its refractive index), a large voltage isrequired to drive currents through a thin film of the material owing tothe additional resistance offered by the silica particles. For thestructure (ii) with the partially-doped PPV however, the resistance isclearly reduced by a considerable amount. Similar drive voltages candeliver comparable current densities through much thicker combinedpolymer film thickness. In the absence of doping, the required voltageis expected to increase as the square of the film thickness so that morethan 50 V will be needed for 1 μA/cm² in structure (ii).

At the same time, no deleterious absorptions in the sub-gap spectralregion of the PPV occurs. This allows the material to be used intransmissive photonic structures.

Example 2 Partial Doping of Organic-Soluble poly(p-phenylenevinylene)and poly(fluorene) Derivatives by Blending with a Second Polymer Bearinga Small Fraction of Dopant Acid Groups

This is an exemplification of the scheme in FIG. 2 a.

Example 2A Preparation of Partially Doped PPV and polyfluorene

Poly(styrenesulfonic acid-co-styrene) copolymer (PSSH-co-PS) as the“second polymer” is blended in [alkoxylphenyl-PPV]-co-[dialkoxyl-PPV](P1) or poly(dialkylfluorene-co-triarylamine) (P2) as host.

This illustrates the generality of the second aspect of the invention.

To prepare the PSSH-co-PS: 0.5 g of polystyrene (4.8 mmol repeat unit)is dissolved with heating into 5 mL anhydrous chloroform in aborosilicate glass reaction flask sealed with teflon-faced siliconerubber septa, and the mixture cooled to −8° C. in a bath of calciumchloride and ice-water. 0.01 mL of chlorosulfonic acid (0.15 mmol) isdissolved into 2 mL of chloroform and then syringed into the PS polymersolution. A white cloudy mixture develops almost immediately. Themixture is warmed to room temperature after 30 minutes and 3 mL of wateris added, and the mixture optionally refluxed. To work up, 40 mL oftoluene is added and the white precipitate is washed and isolated twiceby centrifuge. The precipitate is then purified by dissolving intetrahydrofuran and re-precipitation from toluene. This material isinsoluble in chloroform, methanol or toluene, but soluble intetrahydrofuran from which good quality films can be cast.

The difference Fourier-transform infrared spectrum of a thin film of thePSSH-co-PS cast on silicon substrates shows the asymmetric and symmetricS—O sulfonate bands at 1000-1200 cm¹ and the appearance of the2-adjacent hydrogen wagging vibration at 840-860 cm-¹. This confirmssuccessful sulfonation of PSSH-co-PS with an expected 1-2 mol % of PSSH.Increasing the PSSH content to 50 mol % leads to an intractable materialthat is insoluble in common solvents. Increasing the PSSH contentfurther to near 100 mol % gives a material that is soluble in water andmethanol. The PSSH-co-PS with a low PSSH content less than 10 mmol% istherefore compatible with a range of conjugated polymers, and so can beused to regulate the level of doping in these materials.

Example 2B A Diode Structure Including a Partially Doped PPV orpolyfluorene

To demonstrate that the PSSH-co-PS material with 2 mol % PSSH can beused to improve the electrical conductivity of the conjugated polymerhost, we fabricated diode structures with indium-tin oxide anodes andaluminum cathodes for the following active layers. The drive voltagesfor the selected diode current densities are shown in Table 2.

TABLE 2 Drive voltage Drive voltage required to required to achievecurrent achieve current density of density of Device structure 1 μA/cm²100 μA/cm² (i) Undoped 1.05-μm-thick P1 film 5.5 V 47 V (ii)Partially-doped 1.05-μm- thick P1 2.0 V 33 V film with 1 w/w % ofPSSH-co-PS (iii) Partially-doped 1.05-μm-thick P1 1.1 V  9 V film with10 w/w % of PSSH-co-PS (iv) Undoped 1.65-μm-thick P2 film 9.5 V 40 V (v)Partially-doped 1.65-μm-thick P2 5.5 V 25 V film with 1 w/w % ofPSSH-co-PS

For devices with the 1.05-μm-thick P1 polymer film (that is, structures(i)-(iii)), a substantial reduction in drive voltage, for example from47 V to 9 V at 100 μA/cm², is achieved upon doping with 10 w/w% of thePSSH-co-PS. At the same time, there is no significant loss oftransmission (less than 1% transmission loss) in the sub-gap spectralregion of the host polymer. This is because the ratio of PSSHdopant-to-polymer repeat unit is about 0.5 mol %, and the actual dopinglevel (that is, the ratio of ionized PSSH-to-repeat unit) could be evenlower depending on the ionization potential of the polymer.

For devices with the 1.65-μm-thick P2 polymer (that is, structures (iv)and (v)), a similar reduction is observed, from 40 V to 25 V at 100μA/cm², upon doping with 1 w/v% of the PSSH-co-PS. In this case, dopingwith 10 w/w% of the PSSH-co-PS gives phase separation as evident bycloudy nature of the deposited films.

These examples demonstrate the use of partially-derivatized polymericdopants in a blend to regulate the doping-level and hence improve theelectrical conductivity of conjugated polymer films for photonicstructures.

Example 3 Partial Dopine of an Organic-Soluble poly(fluorene) Derivativeby Partial Derivatization with a Small Fraction of Acid Groups

This is an exemplification of the scheme outlined in FIG. 2 b.

Example 3A Preparation of a Partially Doped poly(fluorene)

In this example, the synthesis and use of a partially sulfonatedpoly(fluorene-co-triphenylamine) (SP2: S denotes sulfonatation) is usedto illustrate the generality of the third aspect of the invention.

To prepare the SP2-co-P2: 0.1 g of P2 (0.25 mmol fluorene repeat unit)is dissolved into 5 ml of anhydrous chloroform in a borosilicate glassreaction flask sealed with teflon-faced silicone rubber septa, and themixture cooled to −8° C. in a bath of calcium chloride and ice-water.Chlorosulfonic acid is dissolved into chloroform to give an equivalentof 0.0025 ml of chlorosulfonic acid (0.037 mmol) per ml of chloroform. 1mL of this solution is then syringed into the P2 polymer solution. Anorange solution is obtained almost immediately. The mixture is warmed toroom temperature after 30 minutes, and 40 mL of acetone is then added toproduce a white precipitate. The precipitate is recovered bycentrifugation and then purified by dissolving in chloroform andre-precipitating from methanol. This SP2-co-P2 material is soluble inchloroform, toluene and tetrahydrofuran but insoluble in methanol andacetone.

The difference Fourier-transform infrared spectrum of a thin film of theSP2-co-P2 material cast on silicon substrates shows (i) the asymmetricand symmetric S—O sulfonic bands at 1355 cm⁻¹ and 1175 cm⁻¹respectively, (ii) the S—O band at 905 cm⁻¹, (iii) the appearance of theone-hydrogen wagging vibration at 860-880 cm⁻¹. This confirmssulfonation of SP2-co-P2 with an expected 5-10 mol % of sulfonic acidgroups.

To demonstrate that this derivatized material has a better electricalconductivity than its parent, we fabricated diode structures withindium-tin oxide anodes and aluminum cathodes for the following activelayers. The drive voltages for selected diode current densities areshown in Table 3.

TABLE 3 Drive voltage Drive voltage for current for current density ofdensity of Device structure 1 μA/cm² 100 μA/cm² (i) Undoped 1.1-μm-thickP2 film 10.5 V 31 V (ii) Partially-doped 1.1-μm-thick P2  4.5 V 21 Vfilm with 1 w/w % of SP2-co-P2

A significant reduction in drive voltage, for example from 31 V to 21 Vat 100 μA/cm in 1.1-μm-thick films, is obtained in the presence ofpartial doping by an equivalent of 0.05 mol %-0.1 mol % of sulfonic acidgroups. At the same time, there is no significant loss of transmissionin the sub-gap spectral region of this P2 polymer.

This example therefore demonstrates the use of a controlled level ofdopant derivatization of the conjugated polymer to improve theelectrical conductivity of conjugated polymer films for use in photonicstructures.

Example 4

Examples 2 and 3 were repeated but with the sulfonic acid polymersubsequently reacted with quantitative amounts of o-nitrobenzylbromidein chloroform or tetrahydrofuran before film formation to give theo-nitrobenzylsulfonate ester as the precursor dopant. The polymer wasthen blended with the host polymer as in Example 2 or used neat (similarto Example 3) in a suitable solvent and formed into a film. Theprecursor sulfonate ester group was then cleaved by UV exposure togenerate the active sulfonic acid group in the film.

Example 5

Examples 2 and 3 were repeated but with the sulfonic acid polymersubsequently reacted with excess of methylating agent such asdiemthylsulfate before film formation to give the methylsulfonate esteras the precursor dopant. The polymer was then blended with smallquantities of a photoacid generator such as diphenyliodonium chloride orother diaryliodonium, triarylsulfonium or other onium salts, and furtheroptionally blended with the host polymer (such as in Example 2) or usedsubstantially neat (similar to Example 3) in a suitable solvent andformed into a film. The polymer film was then exposed to light or heatto cleave the photoacid generator to produce a strong acid with thencleaves the sulfonate ester to the active sulfonic acid.

The invention is not limited to the examples described above.

The invention may include any feature or combination of featuresdisclosed herein either implicitly or explicitly or any generalizationthereof, without limitation to the scope of any of the appended claims.In view of the foregoing description it will be evident to a personskilled in the art that various modifications may be made within thescope of the invention.

1. A method for partial doping of a host conjugated polymer, comprising:blending a host conjugated polymer or a precursor thereof with a secondpolymer to form a polymer blend, the second polymer beingsubstantially-miscible with the first polymer and derivatized with asmall fraction of dopant groups or precursors thereof, and the smallfraction being less than the amount required to fully dope the hostconjugated polymer. 2-35. (canceled)